Communication networks are generally designed as point-to-point networks to interconnect, upon request, selected pairs of terminals from a large plurality of terminals connected to the network. The simplest network capable of connecting N network inlets to N network outlets is an N.times.N crossbar array of switching elements or crosspoints. Although such an array is non-blocking in that any idle inlet is always connectable to any idle outlet regardless of other array interconnections, the crossbar array is not a practical network in many applications due to the prohibitive cost of the large number of array crosspoints.
To reduce cost while still maintaining an acceptably low blocking characteristic, space-division switching networks are frequently designed to include a plurality of stages of switching nodes. The node stages are successively interconnected using a specified interconnection pattern. To achieve the overall switching function of connecting any network inlet to any network outlet, the individual switching nodes are typically selective in that they can connect any one of a plurality of node inputs to any one of a plurality of node outputs in response to control signals defining the desired connections. One simple switching node of this type is a n.times.m rectangular array of crosspoints. As attempts to further reduce network costs continue and, particularly for networks designed for implementation in the photonics domain, the input/output selectivity required of the switching nodes in such multi-stage networks makes the total cost of the switching nodes high.